In this kind of permanent magnet type electricrotary machine, a concentrated winding has been used for a stator winding, and a rare-earth neodymium permanent magnet have been used for a field magnet, thereby having achieved higher efficiency of the rotary machine. Incidentally, vibration and noise components of an electric rotary machine have become larger as a magnetic material has gotten higher strength of a magnetic field, and various countermeasures against such problems have since been taken.
For example, according to a permanent magnet type electric rotary machine as disclosed in JP 2004-48912 A, it has proposed a method of that a plurality of concave portions (they are called as “gap face” for increasing partial gaps between an outer surface of a rotor and an inner surface of a stator) are provided on the outer surface of a rotor, and the respective gap faces are disposed so as to be displaced step by step in the axial direction of the rotor.
On the other hand, in a permanent magnet type electric rotary machine as disclosed in JP 2005-27422 A, it has been proposed that a rotor is provided with a plurality of slits extending from the outer periphery side of a permanent magnet embedded in a rotor toward the outer periphery the rotor.
As a result of adoption of a stator with a concentrated winding and a permanent magnet with a high residual magnetic flux density, efficiency of a permanent magnet synchronous motor has since been dramatically improved. On the other hand, the stator with the concentrated winding brings an increase in pulsation flux in comparison with a stator with a distributed winding, in theory, and furthermore the permanent magnet with the high residual magnetic flux density encourages the increase of the pulsation flux. As a consequence, it brings an increase in vibration and noise of the motor itself, and an intermediate frequency band noise, which is a most grating noise, increases markedly in the case of the motor being particularly assembled into a compressor.
Incidentally, according to JP 2004-48912A (hereinafter, it's called “patent document 1”), it since has adopted so-called skew structure where the plurality of gap faces are provided on the outer surface of the rotor and the respective gap faces are disposed so as to be displaced step by step in the axial direction, so that it is possible that a distribution of a gap magnetic flux density can have a spatial phase difference in the axial direction of the motor. As a result, pulsation components in the magnetic flux density distribution are offset to each other in the axial direction, so that vibration and noise-causing components can be reduced.
On the other hand, according to JP2005-27422A (hereinafter, it's called “patent document 1”), it can provide a structure where a waveform of induced electromotive force can be turned into a sinusoidal wave by adopting the rotor with the plurality of the slits extending from the outer periphery side of the permanent magnet embedded in the rotor toward the outer periphery thereof. Such a structure enables an armature current to be turned into the form of a sinusoidal wave, so that it is possible to reduce a pulsational torque generated by interaction between the induced electromotive force and the armature current.
According to the conventional technology described as above, for example, the technology of the patent document 1 can reduce noise occurring in relatively high frequency bands, and the technology of the patent document 2 can reduce noise occurring in relatively low frequency bands. However, they are not effective sufficiently to reduce the noise in the intermediate frequency band that is a problem picked up herein.
The reason for this is that, in the case of the technology according to the Patent Document 1, in order to cancel out a harmonic component of a low order being longer in wavelength, it needs to adopt a large skew pitch structure, resulting in deterioration of characteristics. In consequence, if an emphasis is placed on maintenance of the characteristics, the skew pitch naturally becomes smaller, thereby providing an effect of reduction only in harmonic components of a relatively high order being shorter in wavelength.
In the case of Patent Document 2, distribution of magneto motive force of the magnet, which is substantially in the form of a square wave, can be improved stepwise by providing the rotor with the slits. However, if an attempt is made to cope with harmonic components of a high order by increasing the number of the slits, this will cause an increase in magnetic resistance of the rotor, resulting in deterioration of the characteristics. For this reason, there are limitations to the number of the slits that can be disposed in a rotor core, so that this method will be effective only for harmonic components of a low order.